1. Field of the Invention
This invention relates to synthesis of diamond whereby in particular, large-sized and plate-shaped diamond can be synthesized with a low cost using an ultra-high pressure apparatus.
2. Description of the Prior Art
The synthesis of diamond single crystal at an ultra-high pressure and high temperature, well known in the art, can be classified into the following two methods.
The first method is called "temperature gradient method" which method comprises using transition metal elements such as Fe, Co, Ni, Cr, Mn, Pt, etc. or alloys thereof as a solvent metal, disposing a carbon source and seed crystal at both the ends of the solvent metal so that the carbon source and seed crystal are not in contact with each other, providing a temperature difference in the solvent metal to maintain the temperature of the seed crystal relatively lower than that of the contact surface of the carbon source and solvent metal, and allowing this assembly to stand at a high temperature and high pressure to grow epitaxial diamond on the seed crystal (U.S. Pat. Nos. 4,034,066 and 4,632,817).
The second method is called "thin solvent film method" and comprises putting a reaction system consisting of a mixture of a non-diamond carbon source powder, solvent metal powder and seed crystal, or a reaction system consisting of a laminate of a non-diamond carbon source plate and solvent metal plate under such a temperature and pressure condition that diamond is thermodynamically stable, and thereby converting the non-diamond carbon source into diamond through the thin solvent film in a short time (U.S. Pat. Nos. 2,947,608; 2,947,609; 2,947,610; and 4,128,625).
As well known in the art, the temperature gradient method of the above described prior art methods is capable of synthesizing a large grain size crystal, but on the other hand, it has the following problems:
(1) The synthesis takes a very long time, so the operation of the apparatus requires a high cost.
(2) Since it is necessary to provide a temperature gradient in a sample chamber, the available volume thereof for the synthesis is small and only a small number of crystals can be synthesized, thus resulting in an increased production cost per one crystal.
In the thin solvent film method, on the contrary, the above described problem (2) can be solved, but complete prevention of spontaneous nucleation is impossible. Therefore, even if the synthesis time is lengthened, it is difficult to synthesize a large grain size crystal and moreover, the synthesized crystal is inferior in quality, for example, contaminated with many inclusions.